The corrosion risk for stainless steel components is not the same in all seawaters, with more failures generally reported in tropical seas. In this study, the influence of biofilm on electrochemical behavior and corrosion resistance of passive films of high-grade alloys was studied in different seawaters, including temperate seawater (France-Brest, North Atlantic Ocean), tropical seawater (Malaysia-Kelatan, Meridional China Sea), and intermediate conditions in terms of temperature (Brazil-Arraial do Cabo, South Atlantic Ocean). The stabilized open-circuit potentials and the polarization behavior of high-grade stainless steels were measured as function of temperature in all the tested field marine stations, providing quantified data and direct comparison on the biofilm-enhanced corrosion risks. Significant differences were measured in tropical and in temperate seawaters in heated conditions above 30°C. In parallel to the monitoring of biofilm-induced depolarization, crevice corrosion of 8 high grades passive alloys was studied with the use of crevice formers specifically developed for tube geometries. Duplex, superduplex, hyperduplex and 6Mo stainless steels tubes have been evaluated together with Ni-based alloys. The corrosion results are discussed regarding the monitored biofilm-induced depolarization measured in the different test conditions.
In natural seawater, microorganisms can fix, grow and develop on practically any surface, including stainless steels [1–3]. The term biofilm is generally used for communities of microorganisms embedded in an organic polymer matrix (e.g. exopolysaccharides), produced by the microorganisms themselves) and adhering to a surface, irrespective of the environment in which they develop. . Stainless steels are widely used for different applications in seawater such as the oil and gas, desalination and marine energy industries. The presence of a biofilm on passive alloys such as stainless steels or nickel-based alloys can strongly enhance the cathodic reactions,  and shift their open-circuit potential (OCP) to the noble direction [6,7]. The resulting increase in OCP is also called cathodic depolarization or ennoblement. The ennoblement in aerated seawater is known to be one of the main factors affecting the risk of localized corrosion of stainless steel and nickel-based alloys, since their critical pitting or crevice potential for protective passive layer breakdown can be exceeded. Among the mechanisms that could explain the so-called ennoblement, some authors mention the role of microbial enzymes, the production of H2O2, and/or low pH inside the biofilm or at the biofilm/stainless steel interface [9–12]. Some recent studies also suggest the role of electroactive bacteria [13,14]. Cathodic depolarization also called electroactive biofilms has been observed in different seas around the world, [7,15–22] including in deep sea ,.